The following documents, mentioned in the course of the description hereafter, illustrate the state of the art:    [1] Horstad I., Larter S. R., Mills N., A quantitative model of biological petroleum degradation within the Brent group reservoir, Org. Geochem., 19, pp. 107-117,    [2] Côme J. M., Experimentation et modélisation de procédés in situ de dépollution par biodégradation aérobie des aquiferes contamines par des hydrocarbures, thesis dissertation, pp. 75-93, april 95,    [3] Z. (Alan) Yu, G. Cole, G. Grubitz and F. Peel, How to predict biodegradation risk and reservoir fluid quality, WorldOil.com April 2002, Vol. 223 No. 4,    [4] B. A. Cragg, K. M. Law, G. M. O'Sullivan, R. J. Parkers, Bacterial profiles in deep sediments of the Alboran sea, western Mediterranean site 976-978, Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 161, p. 433-438, 1999,    [5] Ian M. Head, D. Martin Jones and Steve R. Larter, Biological activity in the deep subsurface and the origin of heavy oil, Nature, Vol. 426 20, Nov. 2003,    [6] B. Carpentier and L. Martin: patent FR-2,830,646,    [7] Larter et al., Biodegradation rates assessed geologically in a heavy oil field, implications for the deep, slow (Largo) biosphere PHENIX, Goldschmidt, 2000,    [8] I. Kowalewski et al., Geochemical study of biodegraded heavy oils of Wabasca sand deposits (Canada), Abstract, Vol. 1, p. 87, 20th International Meeting on Organic Geochemistry, Nancy, 10-14 Sep. 2001,    [9) J. P. Vandecasteele, Microbiologie pétrolière: concepts, implications environnementales, applications industrielles, Vol. 2, chapter 12, pp. 629-675, Collection Publications de l'Institut Frangais du Pétrole, Ed. Technip, 2005.
Biodegradation of an oil consisting of organic matter in form of hydrocarbon-containing molecules is an alteration phenomenon caused by the oxidation of certain hydrocarbon-containing molecules by micro-organisms or bacterial flora. The bacteria consume these hydrocarbon-containing molecules as they breathe and thus get the elements that are essential for their growth and their replication. Biodegradation leads to the formation of a heavy oil that is difficult to produce and commercially less profitable. The study of this phenomenon arouses renewed interest with the development of deep-sea exploration where the presence of heavy oil is a major risk. There are currently few means available for predicting biodegradation risks, whereas the economic need for developing quantitative tools is increasingly great.
Biodegradation thus is a bio-geochemical process that has similarities to a cold combustion operated by micro-organisms. A bacterium capable of degrading hydrocarbon-containing compounds can in fact be considered to be a hydrocarbon-consuming machine using electron-accepting ions (that can be compared to an oxidizer) and rejecting a reducer.
A known model describing the biodegradation of a field from data relative to the Gullfaks field in the North Sea is described in the publication by Horstad et al. [1].
According to this model, filling of a trap with hydrocarbons at a constant flow rate is envisaged. Water saturated with electron acceptors also circulates with a constant flow rate. The field has a simple parallelepipedic symmetry. During filling in the transition zone, the destruction of four n-alkanes is calculated by means of conventional first-order kinetic laws obtained in the laboratory. The mass balance consists of a kinetic hydrocarbon destruction term and the terms of hydrocarbon and electron acceptor supply by convection. The degradation is double, by air breathing and by sulfato-reduction.
In this system, the electron acceptor supply is the limiting factor. The parameters controlling the system are the thickness of the transition zone and the flow rate of the water below the transition zone. The results obtained by means of this type of model appear to be hardly realistic. This is due to the selection of the balances and of the reaction kinetics, the latter being related to the lack of knowledge about the bacterial kinetics and the attack mechanisms developed by the bacteria.
Models integrating a more complex approach of the porous medium and of the material transport are commonly used to simulate biodegradation in shallow polluted layers, notably the SIMUSCOP model (IFP, France), developed on the basis of research work described in reference [2]; it allows to grid (to discretize into cells) in 3D a subsoil and to calculate the biodegradation by air breathing on the BTEX (Benzene, Toluene, Ethylbenzene, Xylene).
The BIO1D software developed by the ECHOSCAN Company (Canada) can also be mentioned, as well as RT3D or PARSSIM1 (Texas University). The documentation relative to these models is available at the following Internet addresses:
BIO1D Model:
http://people.becon.org/˜echoscan/13-22.htm
PARSSIM Model:
http://www.ticam.utexas.edu/˜shuyu/pssProject/
RT3D Model:
http://bioprocess.pnl.gov/rt3d.htm
A bibliography concerning biodegradation simulation within the scope of depollution is also available at the following address:
http://www.nal.usda.gov/wqic/Bibliographies/qb9406.html
In most of these models, only the hydrocarbon-containing molecules of high water solubility are taken into account (BTEX). Oil is present in dissolved form and moves only by diffusion. Sometimes, residual oil moving by convection is also considered. Although the oil saturations involved are not the same as in a petroleum reservoir and although the emphasis is put on the material transports in the aquifer, the problematics is applicable to reservoirs. Unfortunately, application of such models to the sphere of oil prospecting is nearly impossible because of the difference in the time scales (some days to several ten years for pollution problems in contrast with several thousand years for geologic events) and of the difficulty acquiring the input data of the models.
Another method proposed for the petroleum industry is the “BDI” developed by BHP Billiton Petroleum [3]. It is an empirical simplified approach with a limited number of parameters whose result is a biodegradation index that can be converted to API degree by means of a chart.
The relation used for calculating the BDI is as follows [3]:
  BDI  =            ∑              i        =        1            N        ⁢                  (                              (                                          T                i                            -                              T                c                                      )                    ×          Δ          ⁢                                          ⁢                      t            i                          )            /      C      where:
N is the number of stages selected,
Ti is the temperature of the reservoir,
Tc is the critical bacterial activity temperature (usually 65° C.),
Δti is the time in Ma since filling of the trap,
C is an adjustment parameter depending on the basin.
The differences between the BDI method and the method according to the invention are as follows:
The BDI method is purely empirical and requires definition of an adjustment parameter that cannot be a priori determined. Furthermore, this method implies that biodegradation occurs throughout the residence time of the oil in the reservoir (from its filling till now).
Another method, which is the subject of patent FR-2,830,646 [6], allows to model the progressive biodegradation of hydrocarbons trapped in a petroleum reservoir or trap studied, by the action of a bacterial population in an aquifer. This method requires a large amount of input data: data relative to the reservoir studied, concerning the shape and the height of the reservoir, the physical characteristics of the porous medium, the thickness of the transition zone between the hydrocarbons and the water, the composition of the hydrocarbons, of the flow of electron acceptors entering the reservoir and data relative to the bacterial population in the aquifer. This approach is therefore difficult to apply in cases where few data are available.